1. Field of the Invention
The present invention relates to a document reader, an image forming apparatus, and an image processing method which are preferable applied to a scanner, a color digital copying machine, or a complex machine having an image area identification and adjustment function of identifying an image area and adjusting image data.
2. Description of the Prior Arts
Recently, there has been used a color image forming apparatus for forming a color image based on image data related to red (R), green (G), and blue (B) colors obtained from a colored document image or for forming a color image based on image data received from a printer controller (an external device) such as a server or a personal computer (hereinafter, referred to simply as PC). To form an optimum color image by means of this type of color image forming apparatus, it is necessary to identify a photographic image, a screened halftone image, or a character area on the basis of image data of a document and to perform image processing based on a result of the identification before the image formation.
Japanese Unexamined Patent Publication (Kokai) No. Hei09-172544 (1997) discloses an image processing apparatus for identifying a type of a target image in FIG. 2 on page 2. According to the image processing apparatus, an identification means identifies a type of an image based on image data when identifying a type of image data obtained from a document reader (scanner). The image data is processed according to a result of the image type identified by the identification means. When executing a magnification/reduction process, the processing means modifies an identification operation of the identification means. In other words, an identification parameter of the processing means is variable according to reading conditions in the document reader. This enables a precise identification of the type of the target image according to processing conditions or reading conditions of the target image.
Japanese Unexamined Patent Publication (Kokai) No. 2000-134472 discloses a digital image forming apparatus for forming an electrostatic latent image by converting document information to electric signals before developing the electrostatic latent image in FIG. 2 on page 3. The digital image forming apparatus comprises a threshold modification means, which modifies a threshold for classifying dots in the document into a character part and an image part to a character processing side or to an image processing side when setting the threshold. In other words, the setting is made with modifying the threshold for classifying dots into an image and a character. This increases sharpness of the character part and realizes a smooth and natural copy image in the screened halftone image part, which matches the document or a user's intention.
Japanese Unexamined Patent Publication (Kokai) No. Hei07-30752 (1995) discloses an image area identifying apparatus for identifying a type of an image in FIG. 2 on page 2. The image area identifying apparatus comprises a photographic image area identification means, a screened halftone image area identification means, and a character area identification means, wherein the image area identifying apparatus finds an average density for each block from image data, finds a density difference between the average density obtained for each block and an average density of adjacent blocks, and identifies a photographic image area, a screened halftone image area, and a character area on the basis of the density difference. This enables a character image area to be discriminated from a non-character image area from a mixed image area including a character image, a photographic image, and a screened halftone image.
The conventional color image forming apparatuses, however, have the following problems when identifying a photographic image, a screened halftone image, or a character image from image data of a chromatic or achromatic document and performing image processing based on a result of the identification.
(1) In the above patent publications, an image identification parameter is often set on an operating screen easy to operate for a user. When the image identification parameter is set on the operating screen, generally the operating screen is separated into two parts: an operating screen for adjusting a character image and an operating screen for adjusting a non-character image. For example, if a thick gray character with a tinge of some color need be output sharply in solid black, the operating screen separated into two parts requires an operation with visiting the character image adjustment setting screen and the non-character image adjustment setting screen alternately.(2) As stated above, unless the image identification parameter is appropriate according to the document, it is necessary to repeat color adjustment while opening two adjustment setting screens and visiting the screens alternately. Furthermore, inconveniently a user cannot be informed of their adjustment amounts at a time. Therefore, there is the possibility that it becomes hard to predict an output after color adjustment or that it takes a lot of time for color adjustment, thereby deteriorating the operationality at the color adjustment.